19-3 



THE YEAST CELL 



type, galactose -fermenting (aG) culture, was mated with the a- 

 mating-type nonfermenter (ag). Thirteen asci were analyzed from 

 the aGxag hybrid which was demonstrated to be heterozygous for 

 mating type (a /a) and galactose fermentation (G/g). 



Table IS-l 



The Origin of a Nonferaenter of Galactose 



Hybrid i x z 

 G* me ma X G me Ma 



lA a g me Ma 



*Slov/ fermentation. 

 **"F" and "f" symbolize the "flakim 

 characteristic of haplophase cultures. I 

 in Chapter 9 to Chapter 11. 



S" or clumping 

 lee discussion 



The haplophase cultures derived from the asci were designated 

 a or a by mating them with standard mating-type strains derived 

 from Mrak's culture No. 93 of S. cerevisiae. Galactose fermenta- 

 tion was tested by inoculating small tubes of nutrient galactose 

 broth containing smaller inverted tubes. In all these and subse- 

 quent tests the sugar solution was sterilized by filtration through 

 a Seitz filter and added to the autoclaved medium to make a 4 per 

 cent solution of the carbohydrate in the finished medium. Fermen- 

 tation in the positive cultures was recognizable within 24 or 48 

 hours, but the negative cultures were not discarded until they had 

 been under observation for more than three weeks. 



Thirteen asci were obtained by mating an a galactose -fermen- 

 ter by an a galactose -fermenter (fig. 19-1). Mating type segregated 

 in all asci but the zygote was homozygous for galactose fermenta- 

 tion. Another hybrid was made by backcrossing a non-fermenter 

 to the original fermenter parent. Both mating type and galactose 

 fermentation segregated regularly in 6 asci. Finally a hybrid homo- 

 zygous for the nonfermenter alleles was produced by backcrossing 

 a nonfermenter to the original parent. Mating type segregated regu- 



